1. Field of the Invention
Example embodiments of the present invention relate to a circuit and a method of driving a word line, and more particularly to a circuit and a method of driving a word line in a read mode of a nonvolatile memory device embedded on a System-on-Chip (SoC).
2. Description of the Related Art
Generally, it is desirable for a portable device, for example, a mobile terminal, to be light, small and relatively simple. Thus, a System-on-Chip (SoC) technique, which may implement a central processing unit (CPU), a digital signal processor (DSP), a memory, etc. on one chip, has been adapted for use in conventional portable devices, for example, mobile terminals. The memory implemented on a SoC generally includes a nonvolatile memory, for example, a flash memory, as well as volatile memory, for example, a dynamic random access memory (DRAM) or a static random access memory (SRAM). A flash memory may used to safely store data, for example, identification code values, that are encoded and/or decoded for communication security even when a power supply is deactivated due to, for example, an exhausted battery, etc.
In conventional devices, a NOR-type flash memory having a high speed is generally used as the flash memory implemented on a SoC.
Such a flash memory may have three operation modes, e.g., an erase mode, a program mode and a read mode. Voltage levels applied into a cell may differ depending on the operation mode of the flash memory. Thus, a word line voltage, which may be applied into each cell, may be varied depending on the operation mode.
A constant read voltage (for example, about 2.6 V), which may maximize a difference between an on-cell current and an off-cell current, may be used and/or required for a flash memory to accurately detect an on-cell and an off-cell during a read operation.
A read voltage should maintain a substantially constant voltage level that is relatively insensitive to fluctuations of an external power source, temperature fluctuations and process variations; because a fluctuation of the read voltage may cause an incorrect reading of the on-cell and the off-cell. Thus, generally, the read voltage is generated from an internal read voltage generator, which may be relatively unaffected by fluctuations of an external power source, temperature fluctuations and process variations.
A conventional read voltage generator may compare a reference voltage and a read voltage by a constant resistance ratio to maintain a substantially constant level of voltages. However, due to characteristics of circuit operations, a setting time greater than about 200 ns may be required. Thus, in a case where the read voltage is generated after a read command, a conventional read voltage generator may not satisfy a flash memory access time, which may be about 50 ns.
Therefore, a conventional read voltage generator is typically operated during a standby mode and may generate a prescribed read voltage in advance. However, this conventional process may result in increased power consumption during the standby mode.